JPH1167209A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide excellent charge/discharge cycle characteristics and high storage characteristics by arranging a positive electrode using composite particles comprising a base particle and its covering layer as a positive electrode active material, a negative electrode using a material capable of electrochemically absorbing/releasing lithium ions or a lithium metal as a negative electrode active material, and a nonaqueous electrolyte, and forming the base particle and the covering layer each with the specified composite oxide. SOLUTION: A base particle is a composite oxide represented by formula, Lia Cob Mnc M<1> d Ni1-(b+c+d) , [wherein M<1> is at least one element selected from the group comprising B, Al, Si, Fe, V, Cr, Cu, Zn, Ga, and W, 0<a<1.2, 0.1<=b<0.5, 0.05<=c<0.4, 0<=d<0.4, 0.15<=b+c+d<0.7]. The covering layer is a composite oxide represented by formula, Lie Co1-f M<2> f O2 , [wherein M<2> is at least one element selected from the group comprising Mn, B, Al, Si, Fe, V, Cr, Cu, Zn, Ga, and W, 0<e<1.2, 0<=f<0.5].

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery comprising a positive electrode, a negative electrode containing a substance or lithium metal capable of electrochemically occluding and releasing lithium ions, and a non-aqueous electrolyte. More specifically, the present invention relates to improvement of a positive electrode active material for the purpose of providing a lithium secondary battery having good charge / discharge cycle characteristics and good storage characteristics in a charged state.

[0002]

2. Description of the Related Art In recent years,
As a positive electrode active material of a high energy density type lithium secondary battery, a composite oxide of lithium having a high discharge potential and a transition element has attracted attention.

For example, as a composite oxide of lithium and a transition element, LiCoO ₂ (lithium cobaltate) or LiCoO ₂
If NiO ₂ (lithium nickelate) is used, a high energy density battery with a discharge voltage of 4V class can be obtained. LiCoO ₂ is 3.8 to 3.9 V (vs. Li /
Li ⁺ ), which has a particularly high discharge potential, is a positive electrode active material that is already in practical use.

[0004] However, LiCoO ₂ is expensive because the cobalt raw material is scarce and expensive as a resource. Thus, it can be produced using a relatively inexpensive nickel material, close to the discharge potential of LiCoO ₂ 3.6V (v
s. LiNiO having a discharge potential of about Li / Li ⁺ )
₂ is being reviewed.

However, in LiNiO ₂ , the discharge capacity is reduced in a short cycle when charge and discharge are repeated due to the fact that the crystal structure is easily collapsed due to the desorption and insertion of lithium ions during charge and discharge. There's a problem.

[0006] In order to solve the above problems LiNiO ₂ has recently formula _{Li x Mn y Co z Ni 1-} (y + z) O 2
(However, 0.9 <x ≦ 1.2, 0.9 <x ≦ 1.2,
0.0 <y <0.5, 0.0 ≦ z <0.5, 0 <y + z
≦ 0.5. ) Has been proposed to be used as a positive electrode active material (JP-A-8-37).
007). By replacing a part of nickel atoms in LiNiO ₂ with cobalt atoms and manganese atoms, it is intended to improve charge / discharge cycle characteristics.

[0007] However, as a result of investigations by the present inventors, it has been found that a battery using this composite oxide as a positive electrode active material has a problem that when stored in a charged state, the discharge capacity is greatly reduced. This means that during the save,
It is considered that the electrolytic solution is decomposed on the surface of the positive electrode by the catalytic action of nickel in the positive electrode active material.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a lithium secondary battery having good charge / discharge cycle characteristics and good storage characteristics in a charged state.

[0009]

Means for Solving the Problems A lithium secondary battery (battery of the present invention) according to the present invention is a composite comprising a base particle (A) and a coating layer (B) covering the surface of the base particle (A). A positive electrode having body particles as a positive electrode active material, a negative electrode having a substance capable of electrochemically occluding and releasing lithium ions or lithium metal as a negative electrode active material, and a non-aqueous electrolyte; A) is of the formula Li _a Co
^{_{b Mn c M 1 d Ni 1-}} (b + C + d) O 2 [where, M ¹ is B,
Al, Si, Fe, V, Cr, Cu, Zn, Ga and W
At least one element selected from the group consisting of: 0 <a
<1.2, 0.1 ≦ b <0.5, 0.05 ≦ c <0.
4, 0 ≦ d <0.4, 0.15 ≦ b + c + d <0.7. And the coating layer (B) is represented by the formula Li _e Co _1-f M ² _f O ₂ [where M ²
Is Mn, B, Al, Si, Fe, V, Cr, Cu, Z
At least one element selected from the group consisting of n, Ga and W, where 0 <e <1.2 and 0 ≦ f <0.5. ] (Ii).

The coating layer (B) has the formula Li _x CoO ₂
[However, 0 <x <1.2. ] Is most preferable from the viewpoint of improving the charge storage characteristics.

The thickness of the coating layer (B) is preferably 2 μm or less. If the thickness of the coating layer (B) exceeds 2 μm, the charge / discharge cycle characteristics deteriorate.

The negative electrode of the battery of the present invention has a substance capable of electrochemically absorbing and releasing lithium ions or lithium metal. Materials that can electrochemically store and release lithium ions include graphite,
Carbon materials such as coke and organic fired bodies; lithium alloys such as lithium-aluminum alloy, lithium-magnesium alloy, lithium-indium alloy, lithium-tin alloy, lithium-thallium alloy, lithium-lead alloy, lithium-bismuth alloy; Tin, titanium, iron, molybdenum,
Metal oxides and metal sulfides containing one or more of niobium, vanadium and zinc are exemplified.

As the solvent for the non-aqueous electrolyte in the battery of the present invention, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC) and butylene carbonate (BC), and cyclic carbonates are used. Dimethyl carbonate (DM
C), low boiling point solvents such as diethyl carbonate (DEC), methyl ethyl carbonate (MEC), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE) and ethoxymethoxyethane (EME). A mixed solvent is exemplified. The non-aqueous electrolyte solutes (electrolyte salts) include LiPF ₆ , LiPF ₆ , and LiAs.
F ₆ , LiSbF ₆ , LiBF ₄ and LICLO ₄ are exemplified. Instead of a liquid non-aqueous electrolyte, a gel non-aqueous electrolyte or a solid electrolyte can be used.

The positive electrode of the battery of the present invention comprises a base particle (A) composed of a composite oxide (i) having a specific composition, and a composite oxide (ii) having a different composition covering the surface of the base particle (A). As a positive electrode active material. With the base particles (A), good charge / discharge cycle characteristics can be obtained, and by forming the coating layer (B) on the base particles (A), good charge storage characteristics can be obtained. Good charge / discharge cycle characteristics are obtained because the composite oxide (i) constituting the base particles (A) is a stable composite oxide in a charge / discharge cycle.
Also, good charge storage characteristics can be obtained because the nickel-free composite oxide (ii) constituting the coating layer (B)
This is considered to be because there is no catalytic action to promote the decomposition of the electrolytic solution unlike the lithium-nickel composite oxide.

[0015]

EXAMPLES The present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples and can be carried out by appropriately changing the scope of the invention without changing its gist. It is something.

Examples 1 to 6 A positive electrode, a negative electrode and a non-aqueous electrolyte were prepared as described below, and flat lithium secondary batteries A1 to A6 (the batteries of the present invention) were prepared using these. The capacity ratio between the positive electrode and the negative electrode was 1: 1.1. A polypropylene microporous membrane was used as the separator.

[Preparation of positive electrode] In a mortar, LiOH
Ni (OH) ₂ , Co (OH) ₂ , Mn ₂ O ₃ ,
After mixing Al (OH) ₃ with Li: Ni: Co: Mn: Al at an atomic ratio of 1.0: 0.6: 0.2: 0.1: 0.1, the mixture is dried under an atmosphere of dry air. At 750 ° C
After heating for a time, the formula LiNi _0.6 Co _0.2 Mn _0.1 A
Thus, a composite oxide (i) represented by l _0.1 O ₂ was obtained. Then
These composite oxides (i) are pulverized using a jet mill to obtain two types of powder for base particles (A) having a median diameter (particle diameter at a frequency of 50% on a frequency curve) of about 10 μm. Obtained.

In a mortar, LiOH and CoCO ₃
And Li: Co in atomic ratio of 1.0: 1.0 (Examples 1, 4 to 6) or LiOH, CoCO ₃ and Mn ₂ O ₃ , and Li: Co: Mn in atomic ratio of 1. 0: 0.9:
After mixing at 0.1 (Example 2) or 1.0: 0.6: 0.4 (Example 3), 750 ° in a dry air atmosphere.
Heat treatment with C for 20 hours is represented by the formula LiCoO ₂ (Examples 1, 4 to 6), the formula LiCo _0.9 Mn _0.1 O ₂ (Example 2) or the formula LiCo _0.6 Mn _0.4 O ₂ (Example 3). Thus, three types of composite oxides (ii) were obtained. Next, these composite oxides (ii) were pulverized using a jet mill to obtain three kinds of powders for the coating layer (B) having a median particle diameter of about 0.1 μm.

The powder for the base particles (A) and the powder for the coating layer (B) are mixed in a mortar with a molar ratio of 1 in an Ishikawa-type mortar.
0: 1 (Examples 1-3), molar ratio 10: 0.5 (Example 4), molar ratio 10: 3 (Example 5) or molar ratio 10: 5.
After mixing in (Example 6) and heat-treating at 500 ° C. for 2 hours, particles having a particle size of 2 μm or less are removed with a sieve, and the base particles (A) and the coating covering the base particles (A) are removed. Layer (B)
A positive electrode active material powder composed of composite particles consisting of the following was obtained.
The median diameter of these positive electrode active material powders and the thickness of the coating layer were determined by polishing particles encapsulated in a resin to obtain a cross section, and the cross section of the cross section was measured with a scanning electron microscope (SEM).
ron Microscope) and electron probe microanalysis (EPM)
A: Determined using an Electron Probe Microanalyser).
With respect to the five particles, the thickness of the coating layer at three randomly selected points on the cross section of each particle was measured.
The average values t ₁ , t ₂ , t ₃ , t ₄ ,
calculating a t _5, to calculate the average value t of these mean values t ₁ ~t _5, which was used as a thickness of the coating layer.

Each of the positive electrode active material powder, acetylene black as a conductive agent, and polyvinylidene fluoride as a binder are kneaded at a weight ratio of 90: 6: 4, and 2 tons / ton.
After press-molding into a disc having a diameter of 20 mm at a pressure of ² cm ² , it was vacuum-dried at 250 ° C for 2 hours to produce a positive electrode.

[Preparation of Negative Electrode] A rolled sheet of a lithium-aluminum alloy was punched into a disk shape having a diameter of 20 mm to prepare a negative electrode.

[Preparation of Nonaqueous Electrolyte] A nonaqueous electrolyte was prepared by dissolving 1 mol / l of LiPF ₆ in a mixed solvent of ethylene carbonate and dimethyl carbonate at a volume ratio of 1: 1.

Comparative Example 1 LiOH and Co in a mortar
After mixing CO ₃ and Mn ₂ O ₃ at an atomic ratio of Li: Co: Mn of 1.0: 0.5: 0.5, the mixture is heated at 750 ° C. for 20 hours in a dry air atmosphere. And the formula Li
A composite oxide represented by Co _0.5 Mn _0.5 O ₂ was obtained. Next, the composite oxide was pulverized using a jet mill to obtain a powder for a coating layer (B) having a median particle diameter of about 0.1 μm. Example 1 was repeated except that the powder for the coating layer (B) was used instead of the powder for the coating layer (B) comprising the composite oxide (ii) represented by the formula LiCoO ₂ in the preparation of the positive electrode. Similarly, a comparative battery B1 was produced.

Comparative Example 2 In the production of the positive electrode, the formula Li
A comparison was made in the same manner as in Example 1 except that the powder for the base particles (A) composed of the composite oxide (i) represented by Ni _0.6 Co _0.2 Mn _0.1 Al _0.1 O ₂ was used as it was as the positive electrode active material. Battery B2 was made.

Comparative Example 3 LiOH and Ni in a mortar
(OH) ₂ was converted to an atomic ratio of Li: Ni of 1.0: 1.
After mixing at 0 ° C, the mixture was heated at 750 ° C for 20 hours in a dry air atmosphere to obtain a composite oxide represented by the formula LiNiO ₂ . Next, the composite oxide was pulverized using a jet mill to obtain a powder for base particles (A) having a median diameter of about 10 μm. In the preparation of the positive electrode, the powder for the base particles (A) was converted to the formula LiNi _0.6 Co _0.2 M
A comparative battery B3 was produced in the same manner as in Example 1, except that the powder for the base particles (A) comprising the composite oxide (i) represented by n _0.1 Al _0.1 O ₂ was used instead.

(Comparative Example 4) In the preparation of the positive electrode, except that the powder for the base particles (A) comprising the composite oxide represented by the formula LiNiO ₂ prepared in Comparative Example 3 was used as a positive electrode active material as it was. In the same manner as in Example 1, a comparative battery B4 was produced.

<Charge-discharge cycle characteristics and charge storage characteristics of each battery> For each battery, 4.2 at 0.5 mA / cm ² .
After charging to 5 V, 2.75 V at 0.5 mA / cm ²
A charge-discharge cycle test was performed in which the process of discharging to one cycle was one cycle, and the charge-discharge cycle characteristics of each battery were examined. The charge / discharge cycle characteristics were evaluated in charge / discharge cycles (times) in which the discharge capacity was less than 90% of the discharge capacity in the first cycle.
Further, each battery was charged to 4.25 V at 0.5 mA / cm ² and stored at 60 ° C. for 10 days.
The battery was discharged to 2.75 V at cm ² , and the charge storage characteristics of each battery were examined. The charge storage characteristics were evaluated by the ratio of the discharge capacity when stored to the discharge capacity when not stored, that is, the remaining capacity ratio (%). Table 1 shows the results. Table 1 also shows the composition of the base particles and the coating layer of the positive electrode active material used in each battery and the thickness of the coating layer.

[0028]

[Table 1]

From the comparison between the battery A1 of the present invention and the comparative battery B3, it was found that the charge / discharge cycle characteristics were improved by substituting a part of nickel atoms with other elements such as cobalt atoms instead of LiNiO ₂ as the base particles. I understand. From the comparison between the battery A1 of the present invention and the comparative battery B2, it is found that the charge storage characteristics are greatly improved by coating the surface of the base particles (A) with the coating layer (B). From a comparison between the batteries A1 to A6 of the present invention and the comparative battery B1, it is understood that when 50% or more of the cobalt atoms in the composite oxide forming the coating layer are replaced with manganese atoms, the charge / discharge cycle characteristics are reduced. This is because if the substitution amount becomes too large, LiC
This is presumed to be because the crystal structure of oO ₂ changes significantly and the structural stability decreases. Inventive batteries A1, A4 to A
6 and the batteries A2 and A3 of the present invention, the composite oxide forming the coating layer (B) was found to improve the charge storage characteristics.
As (ii), it can be seen that LiCoO ₂ is most preferable. From the comparison between the batteries A1, A4, A5 of the present invention and the battery A6 of the present invention, if the thickness of the coating layer (B) exceeds 2 μm, the charge / discharge cycle characteristics deteriorate. Therefore, the thickness of the coating layer (B) is preferably 2 μm or less. You can see that.

Examples 7 to 16 LiOH in a mortar
, CoCO ₃ and the element M (M is B, Al, Si, Fe,
V, Cr, Cu, Zn, Ga or W) and an oxide or hydroxide of Li: Co: M at an atomic ratio of 1.0: 0.
9: 0.1, and then mixed in a dry air atmosphere at 75
Heat treatment at 0 ° C. for 20 hours to obtain the formula LiCo _0.9 M _0.1
A composite oxide (ii) represented by O ₂ was obtained. Next, these composite oxides (ii) were pulverized using a jet mill to obtain ten kinds of powders for the coating layer (B) having a median particle diameter of about 0.1 μm. Example 1 was repeated except that the powder for the coating layer (B) was replaced with the powder for the coating layer (B) comprising the composite oxide (ii) represented by the formula LiCoO ₂ in the preparation of the positive electrode. In the same manner as in the above, the batteries A7 to
A16 was produced.

Next, a charge / discharge cycle test and a charge storage characteristic test were performed on each of these batteries under the same conditions as above, and the charge / discharge cycle characteristics and charge storage characteristics of each battery were examined. Table 2 shows the results.

[0032]

[Table 2]

From Table 2, it can be seen that B, Al, Si, Fe, V, Cr, Cu, Zn, B, Al, Si, Fe, V, and C are substituted elements of cobalt in the composite oxide (ii) constituting the coating layer (B).
It can be seen that Ga and W can be used. Regarding these substitution elements, it was separately confirmed that when 50% or more of the cobalt atoms in the composite oxide forming the coating layer were substituted with other elements, the charge / discharge cycle characteristics deteriorated.

In the above embodiment, an oxide or a hydroxide was used as a raw material for producing a positive electrode active material.
iNO ₃ , Ni (NO ₃ ) ₂ , Co (NO ₃ ) ₂ , Mn
(NO ₃ ) ₂ , Al (NO ₃ ) ₃ , Fe (NO ₃ ) ₃ ,
Cr (NO ₃ ) ₃ , Cu (NO ₃ ) ₂ , Zn (NO ₃ )
₂ , nitrates such as Ga (NO ₃ ) ₃ , Li ₂ CO ₃ , Ni
CO ₃ , CoCO ₃ , MnCO ₃ , CuCO ₃ , ZnC
Carbonates such as O ₃ , Li ₂ SO ₄ , NiSO ₄ , CoSO
₄ , MnSO ₄ , Al ₂ (SO ₄ ) ₃ , FeSO ₄ , C
r ₂ (SO ₄ ) ₃ , CuSO ₄ , ZnSO ₄ , Ga
₂ (SO ₄ ) _{3 and} other sulfates, CH ₃ COOLi, Ni
(CH ₃ COO) ₂ , Co (CH ₃ COO) ₂ , Mn
(CH ₃ COO) ₂ , Mn (CH ₃ COO) ₃ , Fe
(OH) (CH ₃ COO) ₂ , Cr (CH ₃ CO
O) ₃ , Cu (CH ₃ COO) ₂ , Zn (CH ₃ CO
O) acetates such as ₂ , Li ₂ C ₂ O ₄ , NiC ₂ O ₄ , C
oC ₂ O ₄ , MnC ₂ O ₄ , Al ₂ (C ₂ O ₄ ) ₃ , F
Oxalates such as eC ₂ O ₄ , CuC ₂ O ₄ and ZnC ₂ O ₄ may be used.

In the above embodiment, the base particles (A) are coated with the coating layer (B) by mixing.
A deposition method by ical vapor deposition, a deposition method by firing, a precipitation method by a chemical reaction, or the like may be used.

[0036]

The present invention provides a lithium secondary battery having good charge / discharge cycle characteristics and good storage characteristics in a charged state.

Continued on the front page (72) Inventor Toshiyuki Noma 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (72) Koji Nishio 2-5-2-5 Keihanhondori, Moriguchi-shi, Osaka No. Sanyo Electric Co., Ltd.

Claims (3)

[Claims] 1. A positive electrode having a composite particle comprising a base particle (A) and a coating layer (B) covering the base particle (A) as a positive electrode active material, and electrochemically storing and releasing lithium ions. And a non-aqueous electrolyte comprising a negative electrode having a negative electrode active material or a lithium metal as a negative electrode active material, wherein the base particles (A) have the formula L
^{_{i a Co b Mn c M 1}} d Ni 1- (b + C + d) O 2 [where, M
¹ is B, Al, Si, Fe, V, Cr, Cu, Zn, G
at least one element selected from the group consisting of a and W, 0 <a <1.2, 0.1 ≦ b <0.5, 0.05 ≦
c <0.4, 0 ≦ d <0.4, 0.15 ≦ b + c + d <
0.7. A composite oxide (i) represented by the formula:
The coating layer (B) has the formula Li _e Co _1-f M ² _f O ₂ [where M ² is Mn, B, Al, Si, Fe, V, Cr, C
At least one element selected from the group consisting of u, Zn, Ga and W, where 0 <e <1.2 and 0 ≦ f <0.5. ] A lithium secondary battery comprising the composite oxide (ii) represented by the formula: 2. The method according to claim 1, wherein said coating layer (B) has the formula Li _x CoO
₂ [where 0 <x <1.2. The lithium secondary battery according to claim 1, comprising a composite oxide represented by the following formula: 3. The lithium secondary battery according to claim 1, wherein the thickness of the coating layer (B) is 2 μm or less.